The General Chemistry Demo Lab

Surface Tension

Introduction

Water has many unusual properties as a result of its ability to hydrogen bond. For example, the density of ice is less than that of the liquid and the predicted boiling point is almost 200 degrees C higher than it would be without hydrogen bonding.

The water molecules at the surface of water are surrounded partially by air and partially by water. These surface molecules would be much more stable if they could be in the interior of the liquid where all their hydrogen bonds could be fulfilled (cohesion). Therefore, water normally tends to have the smallest surface possible, i.e. it has a high surface tension, in order to achieve the lowest possible energetic state.

If a solid material more dense than water is placed on the surface of water, what happens next depends on the nature of the material. If the material is hydrophilic ("water loving") it has a surface to which water is attracted. The adhesion of water to the surface of this material coats the surface of the object with water, reduces the surface tension, and causes the object to sink.

If the solid object is hydrophobic ("water fearing"),the unfavorable interactions between the water surface and the object make it difficult to wet the surface. Two forces now come into play -- the energy it would take to overcome this repulsion and the force of gravity. If the force of gravity is strong enough, it will prevail and the object will sink (assuming that the object has a density greater than water). If the gravitational force is less than the surface tension then the object will float on the surface of the water.

Surface tension is what permits water striders and other insects to walk across the surface of water and what enables a needle to float. Of course, the critical feature here is the amount of force per unit area -- put a needle into water end-on instead sideways and the needle will immediately sink.

The Demo

In the demo shown below, sulfur is sprinkled on the surface of water in a large beaker. The sulfur floats because the particles are very small and sulfur is a hydrophobic molecular solid.

When one drop of liquid detergent is added to the beaker without stirring, the sulfur suddenly sinks to the bottom of the beaker.

If you have Apple's (free) QuickTime installed, you can watch a color movie of the demonstration. This movie is 1.05 Mb in size, so it may take a while to download if you have a slow Internet connection.

How it works

Detergents are a class of chemicals that contain hydrophobic (non-polar) hydrocarbon "tails" and a hydrophilic (polar) "head" group. This general class of molecules are called surfactants. Surfactants can interact with water in a variety of ways, each of which disrupts or modifies the hydrogen bonding network of water. Since this reduces the cohesive forces in water, this leads to reduction in the surface tension and our sulfur sinks.

A typical example of a detergent molecule is sodium lauryl sulfate (read that shampoo bottle of yours!). The structure can be represented in several different ways. Notice that in the models the Na ion has been left off because the anion and cation completely dissociate in water:

If you have the MDL Chime plug-in [not available for Mac OS X, sorry...be sure to complain to MDL about this] installed, you can play with this interactive 3-D model of a sodium lauryl sulfate molecule. You can rotate it, change the display features, enlarge/shrink, display solvent accessible surfaces and more...click and play:

When a detergent is placed in water, the long non-polar hydrocarbon tails tend to aggregate because of favorable intermolecular interactions ("like dissolves like" in the interior and ion-dipole interactions at the exterior). The surfactant molecules thereby organize themselves into 3-dimensional spheres called micelles which have a hydrocarbon core and sulfate groups around the outer surface. Here's a 2-D representation:

Without detergent, we can not remove a greasy oily stain from clothing because grease and oil are large, non-polar, hydrophobic molecules. However, the interior core of a micelle is quite greasy just like an oily stain. When we add detergent to our wash water, the oil or grease on our clothes can dissolve in the interior of the micelles and thereby go into solution.

Surfactants can also form other structures. Rather than form a sphere, some surfactants can coat the surface of the water to form a layer one molecule thick, a molecular monolayer. This is shown diagrammatically below:

A good example of a monolayer is oil on water. A small amount of oil can be spread over a large surface of water when the oil is only one monolayer thick! A variety of related structures are also known, particularly in cell walls (lipid bilayers etc.).

There are many, many other Real World examples and applications of surfactants! Here's just one: your body uses surfactants to reduce surface tension in the lungs. The human body does not start to produce lung surfactants until late in fetal development. Therefore, premature babies are often unable to breathe properly, a condition called Respiratory Distress Syndrome. Untreated, this is a serious illness and is often fatal, but administration of artificial surfactants virtually eliminates this health problem.